Many remote sensing apparatus rely on transmission of energy towards a surface or terrain to be probed, followed by subsequent reception of energy reflected therefrom, in order to realize maps of terrain, vegetation or surface topography. In many applications, optical sources find utility in this field of endeavor. Vehicles may be used for bearing the remote sensing apparatus and for traversing the area of study. Ground penetrating radar, various types of other radars and remote sensing devices, gravitometers and other kinds of tools have been employed, in conjunction with other types of apparatus and measurement instruments, and have been used in a broad variety of applications for surface and subsurface characterization.
Each of these tools presents advantages in particularized situations, has technological limitations, may require set-up and analysis time, can include risks and also has associated costs. As a result, a cost-benefit analysis that also reflects the particular aspects of interest may favor coordination of multiple instrument types, including optically-based measurement techniques. Of the various types of remote sensing devices, lidars (instruments employing light detection and ranging) find particular utility in broad area mapping employing aircraft or spacecraft as platforms, generally providing data that is integrated with data from other types of guidance, posture-sensing and position-sensing equipment that tracks path, pitch, yaw, velocity and the like associated with the vehicle or platform. Lidar provides high accuracy data by scanning a laser beam in a direction generally transverse to a trajectory of the vehicle, and senses angle from nadir, intensity and the like via analysis of pulses of optical energy reflected from the surface under study.
However, systems used to date as illumination sources for lidars generally employ multiple discrete optical elements which must be assembled and aligned under conditions of extreme cleanliness and where the alignment involves highly precise coordination of the elements forming the system. Additionally, particulate contamination of any of the surfaces between media in such systems tends to give rise to catastrophic failure of the system, and such precisely-aligned optical systems are susceptible to mechanical misalignment due to temperature-induced changes in dimensions of the various elements and/or also due to vibration. These are not trivial concerns, particularly in applications where the unit is not readily field-serviceable following deployment, and/or in environments subject to relatively high vibration amplitudes, high thrust and/or extreme temperature excursions, such as space-based applications.
Further, the types of pulsed illumination sources utilized in these areas of endeavor have typically included open cavity laser systems, which typically have relatively low electro-optical conversion efficiencies and which do not provide much latitude with respect to wavelength λ or other characteristics of the output energy. These types of lasers are also susceptible to thermally-induced warpage, and are vulnerable to contamination, as well as unwanted vibration-related effects. For at least these reasons, the utility of lasers previously employed in these types of applications suffers limitations.
Several factors influence imaging performance of systems employing lidar for mapping. For example, relatively low propagation loss of the probe beam through the intervening space is but one of several factors influencing choice of wavelength λ for such a system. Another aspect involves the post-return-signal reception processing. In part due to limitations in processor speed, and in part for reasons of discriminating between various return or reflected signals, pulsed laser having repetition rates in a range of up to about ten kiloHertz are preferred for remote imaging deployment.
Many remote sensing systems operate in a range where no more than two optical pulses are provided during an interval starting with pulse transmission and ending with pulse reception, and thus a time difference (“T”) between pulses of twice the distance separating the measurement system from the target (“D”) multiplied by the velocity of light (“c”), or:(2*D)/c≦T,  (Eq. 1)is determined. As a result, the altitude D above the surface to be mapped often determines a maximum practical repetition rate R for the optical pulses being employed. For example, in a scenario involving orbital ranging for characterizing and mapping planetary surfaces at an altitude D of several hundred kilometers, pulse repetition rates are limited to several hundred pulses per second. At closer ranges, of tens of kilometers or less, repetition rates R may be as much as several thousand pulses per second.
An altimeter lidar can be configured such that it sends out a second pulse within any one frequency channel before receiving a returned signal from a first pulse, effectively multiplying the repetition rate R, or, put another way, cutting the interpulse time T given above in Eq. 1.
However, in many cases, these first and second pulses need to be separated in time so that the reflection of the first pulse off the ground will not be confused with reflection of the second pulse from the top of the cloud. The following example will assume that only one pulse is in flight at any one time, that is, a second pulse will not be initiated until after receipt of the first pulse, which is delayed by traveling to and reflecting from the most distant object the system 100 is intended to survey. Results from such an analysis are easily scalable to other schemata which may utilize various known techniques to achieve higher repetition rates.
Since, for Earth, the highest cloud has an altitude D of about twenty kilometers above the ground, the minimum pulse period T is about 133 microseconds, corresponding to pulse rate R which is less than or equal to about 7.5 kiloHertz (see Eq. 3, infra). In this discussion, the repetition rate R is set by a modulator (described later), and the modulator also determines pulse width. Use of pulse widths of ˜one nanosecond results in a high modulation bandwidth, circa one-half GHz.
More rapid modulation rates, such as are used in optical amplifiers for fiber-based information and telecommunication systems, generally require continuously pumped optical sources, and, in turn, the lower repetition rates useful in mapping applications do not favor high pulse rate modulation applied to the optical signal. High repetition rate has been a primary concern with respect to fiber-based optical amplifiers for communications system applications, and that arena has been a focus with respect to development of fiber-based laser amplifiers to date. At the same time, the rate at which present-day lidar equipment can operate, due to cloud detection (aka “cloud folding”), limits practical pulse rates to frequencies less than 7.5 kiloHertz, and which may be as low as 100 Hz.
Solid state laser diodes provide relatively straightforward electro-optical modulation and control capabilities through conventional control of the electrical signals used to drive them. Such laser diodes also provide mechanically robust illumination sources and do not suffer some of the temperature sensitivities and other performance disadvantages that some gas lasers, using open optical cavities, suffer. As semiconductor laser diode sources have become more robust, with increasing power and wavelength λ capabilities, the range of applications for which such laser diodes provide attractive characteristics has also increased.
However, these types of diode lasers do not provide sufficient power per pulse to be used for aerial mapping, unless some form of optical amplifier is provided, in order to boost the energy per pulse. In turn, the power levels required, when achieved via conventional optical amplification systems, also present known effects degrading system performance, such as amplified spontaneous emission (ASE), frequency shifting (optical frequency doubling), spontaneous mode-locking, longitudinal mode beating and thermally-induced lensing, among other phenomena. At sustained high operating power densities, melting or other catastrophic failure of the optical fiber or other optical elements tends to limit the useful lifetime of such laser and amplifier systems to roughly a few seconds of operational life.
A significant result of technological innovations in laser diode sources and in optical amplifiers is that the potential and capability for real-time mapping of topography using systems from airborne platforms is enhanced, where the systems have increased immunity to conventional infirmities. As a result, these capabilities represent strong impetus to incorporate new types of optical sources in lidar-based measurement systems adapted for airborne platform deployment.
In addition to lifetime limitations, optical sources for such systems that have been employed in past also have suffered limitations in flexibility and adaptability of parameters affecting operating characteristics of the system as a whole. These parameters may include capability for wavelength diversity, pulse shape, pulse duration and repetition rate, among others. Typically, any in-situ change in any of these parameters also affects the others, thus greatly limiting the adaptability of a lidar instrument a specific application or narrow range of target reflectivity and distance.
For the reasons stated above, and for other reasons discussed below, which will become apparent to those skilled in the art upon reading and understanding the present disclosure, there are needs in the art to provide more robust optical sources in order to increase useful life of laser light sources intended for laser-based metrology and mapping, while also increasing the performance latitude achievable via employment of such light sources.